1. Field of the Invention
The present invention relates to a stencil printing emulsion ink. More particularly, the present invention relates to a stencil printing emulsion ink of a water-in-oil type which is small in viscosity change when left in the air for a long period of time, and low in an offensive smell and toxic character.
2. Description of the Prior Art
Stencil printing is a method of printing wherein a stencil printing master sheet is perforated and ink is forced to pass through the perforations which were formed in the stencil printing master sheet to print such a body to be printed as a paper.
Heretofore, a water-in-oil type emulsion ink has been used for stencil printing ink, and a volatile solvent, non-volatile solvent, resin, colorant, and surfactant are contained in an oil phase of the water-in-oil type emulsion ink, and an electrolyte, antifreezing agent, and preservative are contained in a water phase of the emulsion ink.
When such a ink is left in the air for a long period of time, volatile components are dispersed from the oil phase and water is dispersed from the water phase. Whereas the amount of the volatile components which are dispersed from the oil phase is decided by the temperature of the emulsion ink, the amount of water which is dispersed from the water phase is decided by the absolute humidity of surroundings. Accordingly, the amount of the volatile solvent and water dispersed from the oil phase and water phase is different depending on the temperature and the absolute humidity. Thus, it was very difficult to obtain an emulsion ink having excellent stability when it was left in the air for a long period of time.
In order to solve the problem, several methods have been proposed wherein the range of boiling point or initial boiling point at distillation of a volatile solvent used for a water-in-oil type emulsion ink is restricted. For instance, in Unexamined Japanese patent publication No. 61-255967, the boiling point of the volatile solvent to be used is restricted in the range of 180.degree. to 270.degree. C. In Unexamined Japanese patent publication No. 5-125320, the initial boiling point at distillation is restricted in the range of 150.degree. to 210.degree. C. Much amount of the volatile solvents used in these methods are dispersed in the air at a temperature of 0.degree. to 40.degree. C., at which stencil printing is carried out, and the amount of water dispersed is increased with the increase of the amount of dispersion of the volatile solvent. Thus the emulsion ink after the dispersion of the volatile solvent and water exhibits, due to the change in the composition of ink accompanied with the dispersion, a behavior similar to the ink comprising an oil phase only, and the ink after the dispersion is liable to cause offset. Also, the content of a pigment in the oil phase in the ink increases, and thus the trend of print concentration to increase up to higher than the print concentration in the case wherein an emulsion ink was not left for a long period of time before its use is noticed. Particularly, when the initial boiling point at distillation is selected to be in the range of 150.degree. to 210.degree. C., the amount of dispersed volatile solvent is increased and a large amount of water is dispersed accompanied with the dispersion of solvent, leading to a remarkable change in the appearance of the print.
Further, when a stencil printing emulsion ink was prepared using a non-volatile solvent without using a volatile solvent in order not to clog perforations in a perforated stencil printing sheet when the ink is left in the printing sheet, only water in the emulsion ink is dispersed to decrease the viscosity of the ink, and thus the bleeding (or blotting) of printed picture images and offset of the ink due to the increase of the amount of ink to be transferred become easy to occur.
Since stencil printing is easy to perform so that the printing has been used in a wide range of applications including office works, it is necessary to suppress an offensive smell of stencil printing ink.